Semiconductor components are generally produced by means of lithography techniques. For this, structures dictated by a mask are projected onto a substrate by means of illumination and projection systems. Generally, light with wavelengths in the UV region is used for this. In the course of the ongoing miniaturization of semiconductor components, wavelengths in the extreme ultraviolet region (EUV) and in the soft x-ray region are being adopted. This spectrum corresponds to wavelengths in the region of 1 to 20 nm.
At these wavelengths, the mask can no longer work by transmission, since there is no transparent material for these wavelengths. One uses masks which work by reflection. In order to assure a reflection, the masks have a multilayer coating. Such multilayers are built up from periodic repetitions, in the most elementary case a period consisting of two layers. Generally, one material of the layer should have the highest possible index of refraction and low absorption, while the other layer material should have the lowest possible index of refraction. The layer with the high index of refraction and low absorption is also known as a spacer, the layer with low index of refraction is also called the absorber. In the EUV range, gem silicon with an index of refraction of 1.0 is used as the spacer and molybdenum with an index of refraction of 0.92 as the absorber. The period thickness and the thickness of the individual layers are chosen in dependency on the operating wavelength, the mean angle of incidence, and the angle bandwidth of the incident radiation so that the reflectivity integrated over the illuminated surface is maximized at the operating wavelength.
Multilayer coatings act as a Bragg reflector, so that one observes a reflectivity depending on the angle of incidence. This becomes noticeable, e.g., in the form of a nonhomogeneous intensity distribution in the pupil of projection optics, the so-called pupil apodization. For it is not technically possible to produce illumination sources and illumination systems providing only precisely one angle of incidence. Furthermore, such so-called coherent illuminations only permit the imaging of gross structures, whereas one can resolve and correspondingly portray finer structures with partially coherent or incoherent illumination systems. Such illumination systems have a finitely opened cone of illumination rays. The pupil apodization can result in imaging errors, such as inhomogeneous intensity when projecting the mask structure onto a substrate, telecentric errors, and structure-dependent or field-dependent limits of resolution (so-called HV differences or CD variations).
EP 1 282 011 A2 shows ways of minimizing the apodization by measures taken for the projection system. The projection objective proposed there for imaging a pattern arranged in an object plane into an image plane by means of electromagnetic radiation from the EUV range has several imaging mirrors provided with multilayer coatings between the object plane and the image plane, defining one optical axis of the projection objective. At least one of the mirrors has a graduated multilayer coating with a layer thickness varying in rotational symmetry to an axis of coating, while the axis of coating is arranged eccentrically to the optical axis of the projection objective. This takes care of the problem of pupil apodization, i.e., the strong fluctuation in intensity over the pupil, since one works with a rotationally symmetrical apodization. As a result, the apodization is almost independent of the field. This is achieved in that the projection objective has two mirrors with centered and graduated multilayer coating, and the two multilayer coatings are appropriately coordinated with each other.
U.S. Pat. No. 6,333,961 B1 concerns itself with lessening the influence of the bandwidth of the source spectrum on the lithographic imaging. The reflective mask is employed in the soft x-ray wavelength region, and the reflection occurs on a multilayer coating. It is proposed to vary the period thickness of the multilayer coating over the depth of the coating. Thanks to this thickness variation, the reflectivity profile becomes broader, depending on the angle of incidence or the wavelength. This has the effect that the lithographic imaging becomes less sensitive to fluctuations in the angle of incidence and the wavelength. The thickness variation can be continuous or occur in stages over the depth of the coating.